Moisture curing polyurethane based hotmelt adhesive compositions (HMMC) are well known in the art. HMMC adhesives are typically based on isocyanate-terminated prepolymers. The NCO-terminated prepolymer reacts in the presence of water, e.g. air humidity, to form urea-based adhesive bonds, while CO2 is eliminated. Suitable NCO-terminated prepolymers may be obtained by reacting isocyanate-reactive polymers such as a polyol with a molar excess of multifunctional isocyanate agents such as a monomeric diisocyanate. However, significant amounts of monomeric diisocyanate normally remain in the NCO-terminated prepolymer, which leads to health issues. Therefore, the residual amount of monomeric diisocyanate in the NCO-terminated prepolymer has to be reduced to less than 1% or lower. This can be done, e.g., by stripping, which is a costly and time consuming additional purification step. In view of this, attempts have been made to find more efficient and convenient procedures to reduce the amount of residual monomeric diisocyanate in NCO-terminated prepolymers.
WO 2003/055929 A1, WO 2003/051951 A1, WO 2003/066700 A1, WO 2003/006521 A1, WO 2001/040342 A1 or WO 2005/033164 A1 describe the preparation of reactive polyurethanes having a low content of monomeric diisocyanate by using asymmetrical diisocyanates such as 2,4′-MDI.
EP 1338635A discloses a polyurethane compositions with low content of diisocyanate monomers obtained by reacting at least one isocyanate-reactive monomer with an excess of molten monomeric diisocyanate to produce at least one reaction product having a maximum of 0.5 wt % of free isocyanate groups.
EP 2894181A describes a foaming polyurethane composition with low residual monomeric diisocyanate obtained by blending isocyanate component, a propellant, monofunctional alcohol, a catalyst, a stabilizer and one or more polymeric polyols.
US 2004/0162385 describes the production of a reactive polyurethane by reacting 2,4-MDI with propyleneglycol and then subsequently adding polyester polyol and 4,4′-MDI.